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Original citation: Birrell, Stewart A and Haslam, Roger A.. (2010) The effect of load distribution within military load carriage systems on the kinetics of human gait. Applied Ergonomics, Vol.41 (No.4). pp. 585-590. Permanent WRAP url: http://wrap.warwick.ac.uk/53195 Copyright and reuse: The Warwick Research Archive Portal (WRAP) makes this work of researchers of the University of Warwick available open access under the following conditions. Copyright © and all moral rights to the version of the paper presented here belong to the individual author(s) and/or other copyright owners. To the extent reasonable and practicable the material made available in WRAP has been checked for eligibility before being made available. Copies of full items can be used for personal research or study, educational, or not-for-profit purposes without prior permission or charge. Provided that the authors, title and full bibliographic details are credited, a hyperlink and/or URL is given for the original metadata page and the content is not changed in any way. Publisher’s statement: “NOTICE: this is the author’s version of a work that was accepted for publication in Applied Ergonomics. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Applied Ergonomics, Vol.41 (No.4). pp. 585-590. http://dx.doi.org/10.1016/j.apergo.2009.12.004 A note on versions: The version presented here may differ from the published version or, version of record, if you wish to cite this item you are advised to consult the publisher’s version. Please see the ‘permanent WRAP url’ above for details on accessing the published version and note that access may require a subscription. For more information, please contact the WRAP Team at: publications@warwick.ac.uk

The Effect of Military Load Carriage on Ground Reaction Forces

Technical Note

Stewart A Birrell 1

Robin H Hooper 1

Roger A Haslam 1

1Department of Human Sciences, Loughborough University, Leicestershire, LE11 3TU,

UK

Keywords: Load Carriage, Ground Reaction Force, Military, Gait, Rifle Carriage

Abstract

Load carriage is an inevitable part of military life both during training and

operations. Loads carried are frequently as high as 60% bodyweight, and this increases

injury risk. In the military, load is carried in a backpack (also referred to as a Bergen) and

webbing, these combined form a load carriage system (LCS). A substantial body of

literature exists recording the physiological effects of load carriage; less is available

regarding the biomechanics. Previous biomechanical studies have generally been

restricted to loads of 20 and 40% of bodyweight, usually carried in the backpack alone.

The effect of rifle carriage on gait has also received little or no attention in the published

literature. This is despite military personnel almost always carrying a rifle during load

carriage. In this study 15 male participants completed 8 conditions: military boot, rifle,

webbing 8 and 16 kg, backpack 16 kg, and LCS 24, 32 and 40 kg. Results showed that

load added in 8 kg increments elicited a proportional increase in vertical and

anteroposterior ground reaction force (GRF) parameters. Rifle carriage significantly

increased the impact peak and mediolateral impulse compared to the boot condition.

These effects may be the result of changes to the vertical and horizontal position of the

body’s centre of mass, caused by the restriction of natural arm swing patterns. Increased

GRFs, particularly in the vertical axis, have been positively linked to overuse injuries.

Therefore, the biomechanical analysis of load carriage is important in aiding our

understanding of injuries associated with military load carriage.

Introduction

Military mission requirements often depend on personal mobility. In these

situations personnel carry their own equipment, usually in a backpack (Bergen) and

webbing, so forming a load carriage system (LCS). A rifle is also carried on most

occasions when marching. The study of ground reaction forces (GRF) during load

carriage can provide relevant information about the mechanisms of gait, and provide a

measure of the impact forces acting on the foot. It is therefore essential in the

understanding and prevention of lower extremity injuries [1].

Research investigating the effect of load carriage on GRFs and gait is not widely

represented within the literature [1,2,3,4,5,6]. However, conclusions drawn confirm, that

as would be expected, both vertical and anteroposterior GRFs produced during gait

increase when load is applied to the body. However, the proportionality or rate of this

increase has been debated within the literature. The majority of research suggests that the

increase in vertical and anteroposterior GRFs is directly proportional to the applied load

[2,3,4]. These studies suggest that 1 kg of added load equates to approximately a 10 N

increase in force. Other studies suggest that protective mechanisms, such as an increase

in double support or decreased walking speed, are activated when carrying heavy loads in

an effort to reduce stresses placed on the lower extremities [1,5]. Finally, changes to the

GRF parameters of the mediolateral axis have been found to be insignificant [1,4,6].

The primary aim of this research was to examine the effect of progressive 8 kg

increments in carried load on GRF parameters. This would help establish base-line GRF

data for load carried using the U.K ‘90 Pattern LCS, and investigate heavy military load

carriage. The study design allowed other factors to be investigated including the effect of

changing the load distribution, and also the potential effects of rifle carriage on GRF

parameters.

Technical Description

Fifteen male participants volunteered for the study (mass 83.2 kg ± 10.0 S.D.,

height 178.8 cm ± 5.4, age 27.8 years ± 7.0). In order to comply with the granted ethical

approval and for the % bodyweight carried to be deemed acceptable, each participant had

to weigh over 70 kg. All participants also had previous experience of carrying backpacks,

and were rear-foot strikers. A verbal and written explanation of the study was given, after

which a health screen questionnaire was completed. Informed consent was obtained from

all participants before commencing the trial.

A Kistler™ force plate (Type 9286A, dimensions 60 x 30 x 5 cm) was used in

conjunction with a Coda™ Mpx30 Motion Analysis System to obtain GRF data. Eight

channels of kinetic data were sampled by the force plate at 400 Hz. This raw data were

then processed via A/D converters situated in the Coda Mpx30 and outputted into

CODAmotion v6.64 software. The data were then exported to Microsoft™ Excel for

analysis. The force plate was embedded in an 8.4 m walkway. This gave adequate

distance before and after the force plate to achieve a natural gait pattern. To measure the

walking speed of the participants three pairs of infra-red photoelectric cells (Brower™

SpeedTrap II) were used placed 1.5 m apart from each other. One set recorded speed on

approach to the force plate and the other after the force plate. Both speeds had to be

within the desired range thus limiting the potential for acceleration or deceleration that

would affect the GRFs produced.

The load was carried using a standard issue UK military ‘90 Pattern Short Back

Bergen and PLCE (Personal Load Carrying Equipment) waist webbing, which, when

worn together form a LCS. A replica SA80 assault rifle, weighing 2.1 kg, was also

carried in certain conditions. Participants also wore standard issue military leather boots

and woollen socks throughout the duration of the study.

Each participant completed all 8 conditions (table 1), with 10 successful trials in

each condition. The force data were sampled at 400 Hz and the target speed throughout

was 1.5 m.s-1 (± 5%). A trial was deemed successful if the speed was attained, the

participant’s dominant foot struck cleanly on the force plate and if a natural gait pattern

was maintained. To ensure participants had familiarised themselves with the load and

walking speed an unlimited number of practice walks were allowed. The order the

participants completed the conditions were randomised.

Insert Table 1 Here

The participant’s kinetic data were normalised and expressed as Newton’s per

unit body mass (N.BM-1). Data from the boot condition were normalised to bodyweight

(including clothes and boots), the other conditions to system weight (this is the weight of

the rifle added to that of the participant). All data are expressed as N.BM-1 but as

explained above this may either be the weight of the participant alone, or with the rifle.

The primary aim of the study was to examine the effects that small, incremental

load increases of 8 kg have on selected GRF parameters. For this reason the boot and

backpack condition were excluded from this particular section of the analysis (table 1).

This is because the rifle condition was considered a more suitable control to the boot

condition, as a rifle would be carried during each loading conditions. The backpack

condition was also excluded, thus eliminating the issue of having two conditions where

the carried load totalled 16 kg. To assess the effects of carried load on GRF parameters a

one-way MANOVA was undertaken. To determine significance between the conditions a

Bonferroni corrected pairwise comparisons were also conducted. A Paired Student t-test

was conducted to assess significance with rifle carriage and changing load distribution.

For these comparisons the boot and backpack condition were re-introduced. Significance

was accepted at the level of p≤0.05 and all statistical testing was conducted using SPSS

v12.0.

Discussion

Effect of Load

Increasing carried load has a significant overall effect on all the GRF parameters

measured (table 2). In addition, the pairwise comparisons revealed that all parameters,

with the exception of mediolateral impulse, significantly increased with incremental

increases of 8 kg. An increase in load has been shown to increase GRF consistently

within the literature [1,2,3,4,5,6]. An increase in stance time was also observed, which

has been observed within the literature [2,5]. Numerous studies [1,4,6] have found

changes to the mediolateral GRF parameters to be insignificant, and others did not even

report the data. Results from this current study go against this idea as a significant

increase in total mediolateral impulse was observed with load. The increase in

mediolateral impulse observed here may be linked to a decrease in stability. This may be

caused by the continual shift (in both the vertical and horizontal direction) of the body’s

centre of mass (CoM) further away from its neutral position when load is added.

Research has shown that the less the CoM is displaced the greater the static stability of an

individual when carrying load [7].

Insert Table 2 Here

As highlighted previously, the literature on the proportionality of the increase in

GRF parameters with applied load is contradictory. Results from the present study

support the hypothesis that increases in vertical and anteroposterior GRF with applied

load represent a linear relationship when walking at 1.5 m.s-1, even when heavy loads of

40 kg are carried. This suggests that the increase in force is predominantly due to the

static effect of the load rather than changes in acceleration of the system [2]. Figure 1

shows the linear increase in measured force against carried load.

Insert Figure 1 Here

High magnitudes or volumes of impact forces, like those experienced during load

carriage or running, are a major risk factor for overuse injuries. In particular, stress

fractures of the tibia and metatarsals and knee joint problems [4,8,9]. Military recruits can

cover up to 11 km per day, which is equivalent to around 9,000 impacts [10]. For this

reason it may be advantageous to have the ability to accurately predict the forces

produced when heavy loads are carried over known distances. Establishing a dose-

response relationship for distance marched and load carried may be feasible. This would

require the knowledge of the maximum stress or strain that can be placed on a bone or

joint before stress fractures or joint degeneration are likely to occur. It would then be

possible to calculate the number of impacts made and accurately estimate the peak force

produced during these impacts. This may allow prediction of the number or severity of

overuse injuries sustained during a forced march by military personnel. Other factors

need to be taken into account such as prior exposure to marching and previous injury.

However, training regimes could be adapted to reduce the risk of overuse injuries, and

theoretical maximum distances marched while carrying specific loads could be drawn up.

These distances or loads could then increase as training advances, as soldiers become

more used to the physical activity and as increases in bone mineral density of the lower

limb occur. Using linear regression analysis to calculate the increase in impact peak force

with load gave this equation: Impact Peak = (0.013 x Load) + 1.223. Load is measured in

kg and the values for impact peak are expressed as N.BM-1. The proportion of variation

which can be explained by this equation (R2) is 0.780 or 78%.

Rifle Carriage and Load Distribution

As mentioned in the introduction the study design allowed the effect of rifle

carriage and changes to load distribution, with their subsequent affect on GRF, to be

analysed. The following section will highlight differences found with the current study,

however, more detailed analysis is needed with future research. The effect of rifle

carriage was examined by comparing the Boot and Rifle condition and, load distribution

by comparing the Webbing 2 and Backpack conditions (table 1). Results in table 3 show

that the rifle condition exhibited a greater impact peak, maximum propulsive force and

mediolateral impulse, while decreasing the force minimum compared to the boot

condition. The most likely mechanism behind these changes to GRF parameters with rifle

carriage is either the restriction of natural arm swing patterns, or the load of the rifle

being added to the anterior of the body.

Insert Table 3 Here

Carrying 16 kg in the webbing compared to the backpack lead to an increased

impact peak in the vertical axis, and a reduction in stance time (table 3). Higher impact

forces observed in the webbing condition, may be due to a larger component of the

weight being over the striking foot at the time of initial contact. This is supported by

other research that states when the CoM is shifted anteriorly, the force at heel strike is

increased [11]. Stance time was also significantly longer when carrying the backpack

compared to webbing. This occurrence has been observed before [2,6], with a backpack

showing a trend for longer stance times than with a double-pack (load distributed around

the anterior and posterior of the trunk). Reason for this increase may be due to the extra

time it takes to shift the CoM over the base of support, or an increased need for stability.

Another factor may be as a result of increased dampening or flexion of the lower limb.

Conclusions

This study aimed to examine the effects of progressive increments in carried load

on GRF parameters. Results from the study suggest that both vertical and anteroposterior

GRF parameters increase proportionally when load is added in 8 kg increments to a UK

standard issue ‘90 Pattern LCS. This increase is observed even when heavy loads of 40

kg are carried. Unlike many other studies significant increases in force generated in the

mediolateral axis was also observed with increasing load. This may suggest a decrease in

stability as greater loads are carried.

A new finding for this field of research is the effect of rifle carriage on GRFs.

Rifle carriage caused an increase in the impact peak, maximum propulsive force and

mediolateral impulse while decreasing the force minimum. These effects may be due to

the forward shift in the CoM or more likely due to the restricted arm movements while

carrying a rifle.

Acknowledgements

This work was sponsored by the UK Ministry of Defence under contract with Dstl.

References

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biomechanics of load carriage. Technical Report No T00-17, U.S Army Research

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[2] Kinoshita H. Effects of different loads and carrying systems on selected

biomechanical parameters describing walking gait. Ergonomics 1985; 28: 1347-1362.

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carriage upon the lower limb. Human Movement Science 1999; 18: 693-700.

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carried by soldiers: Combined analysis of four studies on maximal performance,

physiology, and biomechanics. Technical Report No TR-02/010 US Army Research

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temporal factors in level grade backpacking. Journal of Human Movement Science 1991;

21: 167-181.

[6] Lloyd R, Cooke CB. Kinetic changes associated with load carriage using two

rucksack designs. Ergonomics 2000; 43: 1331-1341.

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Presentation: Natick Soldier Systems Center, Natick, MA. 2004.

[8] Cavanagh P, Lafortune MA. Ground reaction forces in distance running. Journal of

Biomechanics 1980; 13: 397-406.

[9] Knapik J. Discharges during US Army basic training: Injury rates and risk factors.

Military Medicine 2001; 166: 641-647.

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0

0.5

1

1.5

2

0 8

Forc

e (N

.BM

-1)

Impact P

B)

-0.5

-0.25

0

0.25

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Forc

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Ma

Figure 1: Changes to

increase in load, erro

A) Vertical

16 24 32 40

Load (kg)

eak Force Min Thrust Max

Anteroposterior

16 24 32 40

Load (kg)

x Brake Force Max Prop Force

mean vertical (A) and anteroposterior (B) GRF parameters with

rs bars are represented by the standard deviation of the data.

Table 1: Description of the conditions used during the trial and total load carried.

Condition Description Load Boot Wearing non-restrictive clothes and military boots 0 kg Rifle As Boot, but carrying a replica SA80 rifle 0 kg

Webbing 1 As Rifle, with the addition of 8kg webbing 8 kg Webbing 2 As Webbing 1, increasing load to 16 kg 16 kg Backpack As Rifle, with the addition of 16 kg Bergen 16 kg

LCS 1 As Rifle, carrying 8 kg webbing and 16 kg Bergen 24 kg LCS 2 As Rifle, carrying 16 kg webbing and 16 kg Bergen 32 kg LCS 3 As LCS 2, with addition of 8 kg in the Bergen 40 kg

Table 2: Results showing changes to mean GRF parameters with the addition of 8 kg increments of load from 0 to 40 kg, standard deviation in

parentheses. Significance derived from the overall effect of load on selected parameter, * indicates significance (p≤0.05). Forces are measured in

(N.BW-1), Impulses and Rates in ((N.BW-1).s) and Time in (s).

GRF Parameter Condition Level of Rifle Webbing 1 Webbing 2 LCS 1 LCS 2 LCS 3 Significance

Impact Peak 1.226 (0.08) 1.327 (0.08) 1.443 (0.09) 1.541 (0.11) 1.650 (0.11) 1.763 (0.13) p ≤ 0.001 * Force Minimum 0.602 (0.05) 0.644 (0.05) 0.697 (0.06) 0.741 (0.06) 0.795 (0.04) 0.854 (0.05) p ≤ 0.001 * Thrust Maximum 1.205 (0.08) 1.326 (0.09) 1.434 (0.09) 1.571 (0.09) 1.645 (0.10) 1.721 (0.12) p ≤ 0.001 *

Max Braking Force -0.287 (0.04) -0.306 (0.06) -0.334 (0.04) -0.356 (0.06) -0.368 (0.06) -0.399 (0.07) p ≤ 0.001 * Max Propulsive Force 0.222 (0.03) 0.246 (0.04) 0.266 (0.03) 0.289 (0.04) 0.300 (0.03) 0.321 (0.03) p ≤ 0.001 *

Vertical Impulse 1.076 (0.05) 1.191 (0.07) 1.288 (0.06) 1.411 (0.08) 1.492 (0.09) 1.595 (0.10) p ≤ 0.001 * Mediolateral Impulse 0.043 (0.01) 0.048 (0.01) 0.050 (0.01) 0.052 (0.01) 0.053 (0.01) 0.056 (0.01) p = 0.043 *

Stance Time 0.663 (0.02) 0.674 (0.02) 0.676 (0.02) 0.689 (0.02) 0.689 (0.02) 0.692 (0.03) p = 0.003 *

Table 3: Results showing changes to selected mean GRF parameters for rifle carriage (boot and rifle condition) and load distribution (webbing 2

and backpack condition), standard deviation in parentheses. * indicates significant difference between conditions (p≤0.05).

GRF Parameter Condition Level of Condition Level of Boot Rifle Significance Webbing 2 Backpack Significance

Impact Peak 1.203 (0.09) 1.226 (0.08) p = 0.029 * 1.443 (0.09) 1.409 (0.10) p = 0.010 * Force Minimum 0.622 (0.06) 0.602 (0.05) p = 0.018 * 0.697 (0.06) 0.703 (0.05) p > 0.05 Thrust Maximum 1.212 (0.09) 1.205 (0.08) p > 0.05 1.434 (0.09) 1.443 (0.09) p > 0.05

Max Braking Force -0.286 (0.05) -0.287 (0.04) p > 0.05 -0.334 (0.04) -0.338 (0.05) p > 0.05 Max Propulsive Force 0.215 (0.03) 0.222 (0.03) p = 0.011 * 0.266 (0.03) 0.264 (0.04) p > 0.05

Vertical Impulse 1.082 (0.06) 1.076 (0.05) p > 0.05 1.288 (0.06) 1.297 (0.07) p > 0.05 Mediolateral Impulse 0.040 (0.01) 0.043 (0.01) p = 0.025 * 0.050 (0.01) 0.047 (0.01) p > 0.05

Stance Time 0.662 (0.02) 0.663 (0.02) p > 0.05 0.676 (0.02) 0.687 (0.02) p = 0.002 *